this provides better compatibility for older Visual Studio versions;
stdint.h is a part of C99.
this is consistent with the core libwebp library:
8fbb9188 prefer webp/types.h over stdint.h
Change-Id: I777cad70d9a407646c4193db9ca0f09e65d88735
It's self contained apart from a dependency on src/webp/types.h and src/dsp/cpu.h
For now it's only set up as an internal library, not an installable one.
Webp doesn't depend on it yet, the code is only duplicated.
Change-Id: I752799894f9d4105d0d296ddebd9f9641181a1ec
